In order to prevent unauthorized copying of original printed documents, techniques which add specific image information such as background pattern when the original document is printed have been used. This specific image information is often referred to as “copy preventing information” or “anti-copy information”.
For example, when image data is fed from an application running on a personal computer to a printer for printing out the image data from the printer, specific image information which prevents the copying of the image to be printed is fed to the printer together with the above image data, and embedded in somewhere of the image to be printed for printing out. When trying to copy the printed image in which the specific image information is embedded, the copy machine which has detected the specific image information will not output the copied image or output a copy sheet having thereon characters such as “Copy Prohibited”.
The above-mentioned specific image information may be embedded in printed image in the way that it cannot be recognized by the user, or may be embedded, for discouraging the copying, in the way that it can be recognized by the user that it is the specific image information. In either case, the copy machine detects the presence of the specific image information in the image information when the image information in which the specific image information is embedded is copied and conducts predetermined processing, for example, prohibition of the copying per se or outputting of a copied image having characters such as “Copy Prohibited” in response to a detection result.
In connection with the technique for prohibiting unauthorized copying, a copy machine which is capable of easily making an original document bearing specific pattern thereon is disclosed in, for example, Japanese Laid-Open Patent Publication No. H7-38737. The copy machine comprises a generating means for generating (a) specific mark(s) for reproducing the image with adding the specific mark(s) to an original document image; a mark detecting means for detecting the presence of the specific mark(s) in read original document image; and a control means which conducts the copying operation which is different from usual copying operation if the specific mark in the original document image is detected by said mark detecting means.
A copy machine which reproduces an image which makes it possible to detect the specific mark(s) in an easy and precise manner is disclosed in Japanese Laid-Open Patent Publication No. H7-36325. This copy machine has a capability of reproducing an image to which (a) specific mark(s) is/are added by a mark adding means. The mark adding means adds specific marks, each comprising a plurality of concentric circles having different diameters, to the original document image.
Besides the technologies of Japanese Laid-Open Patent Publication Nos. H7-38737 and H7-36325, the methods of adding the specific image information include a method of mixing information in a frequency area as is the case of a digital watermark, a method of adding a bar-code, etc., to a non-image area, a method of adding information such as a watermark to an image area, etc.
With regard to a system that embeds specific image information in an image to be printed for printing out in accordance with a conventional art, in the case of a digital watermark, when additional information is embedded in a region that is difficult to be decoded, it is problematic that the embedded information cannot be utilized for some types of original image information. In the case of a method of adding a bar-code, etc., to a non-image area, it is problematic that an image area is narrowed, and in the case of a watermark, it is problematic that the printed document becomes less recognizable and an amount of toner consumption is increased.
In a system of such conventional art, when a printed image is read to detect whether specific image information is included, misrecognition is increased depending on characteristics of the printed image. For example, even though the specific image information is not printed on an original document, when the original document is copied, a characteristic of the original document may coincide with a characteristic of the specific image information and stop the print operation. Even if a characteristic of the original document does not coincide with the specific image information, when the original document is repeatedly copied many times, scratches may be formed and so-called trash or foreign particles may be printed, which may coincide with a characteristic of the specific image information and stop the print operation, although the specific image information is not printed on the original document. The detection accuracy of the specific image information is varied depending on usage patterns of a user and the information may not be detected in some cases.